1. Field of the Invention
The invention refers to an apparatus for controlling the flow separation line of rocket nozzles for reducing the side loads on said nozzles.
2. Description of the Related Art
During start-up and stop transients in sea level rocket engines significant dynamic and static loads, often called side loads, occur. These loads are generally attributed to the disordered flow characteristics of the flow during flow separation. These side loads usually limit the size of the nozzle that can be used and thereby the performance of the rocket engine.
There are principally two ways of operating a rocket engine nozzle with respect to flow separation:
a) The first way and the way all nozzles starting at sea level are operated today is that they are designed to operate full flowing, that means without flow separation during nominal operation. However, during the engine start-up there is a short period of side loads when the nozzle during transient conditions in not full flowing. This period of time is typically less than two seconds and the side loads will disappear when the pressure of the combustion gases in the nozzle reaches its nominal value.
b) The second way which is not used today is to have a continuous flow separation during steady-state operation. The flow separation in the nozzle will progress until the rocket reaches an altitude where the pressure in the atmosphere has decreased to a level allowing the nozzle to be full flowing.
The invention intends to achieve control of the side loads both under transient conditions and under steady-state conditions.
Nozzles for liquid propellant rocket engines often operate at conditions where the main jet exhaust into a non-negligible ambient pressure. Examples of such rocket engines are large liquid propellant sea level rocket engines for boosters and upper stage engines for multi-stage rockets.
The side loads that are generated on such nozzles are generally of such a magnitude that they present constraints for the design of the components carrying the nozzle. These constraints result in higher weight of the nozzle itself and the components carrying the nozzle. The largest possible area ratio that can be used is furthermore limited by requirement of full flow function under steady-state conditions.
The final consequences of the side loads are constraints on the overall performance-to-weight ratio of the nozzle and the subsequent limitation of the amount of payload that can be delivered into orbit by the rocket launcher.
For eliminating the drawbacks of prior art nozzles a number of techniques have been suggested which all have turned out to have themselves significant drawbacks in various respects. The drawbacks refer to function, performance, cooling and reliability.
Thus, traditional bell-shaped nozzles have a limit function and substantial start and stop transient loads. A dual bell nozzle also suffers from severe transient side loads.
A known technique for reducing the side loads of a bell-shaped nozzle is to provide the nozzle with trip rings which reduces the side loads, but the trip rings will induce a performance loss when the nozzle is full flowing and it is also difficult to cool said rings.
Another known technique for reducing the side loads of a nozzle is to provide said nozzle with an exit diffusor at the nozzle end, which reduces the effective area ratio of the nozzle. The exit diffusor increases the weight of the nozzle and said exit diffusor must be dropped at high altitude, which require means for active control and moving parts. Also the heat load on the exit diffusor is very high.
Yet another known technique for reducing the side loads of a nozzle is to place an ablative insert on the inside of the nozzle wall, which insert is ablated as the rocket engine burns and is totally gone when the nozzle reaches high altitude. The drawbacks with this technique is that the nozzle will be heavier and one cannot guarantee that the insert will ablate evenly around the circumference.
Yet another known technique for reducing the side loads of a nozzle is to provide fens stretching in the axial direction of the nozzle on the inside of the nozzle forcing the flow separation to be more axi-symmetrical. The flow at the wall will be divided in pockets between said fens. Hereby large areas of different wall pressure causing side loads are avoided. The draw-backs with this technique is that the nozzle will be heavier and the fens will be exposed to an extreme heat load, since they are mounted normal to the nozzle wall and extend into the main jet. Furthermore, they are difficult to cool.
The object of the invention is to eliminate the above-mentioned drawbacks of the prior art.
According to the invention this is achieved in that the inside of the nozzle has circumferentially regularly spaced areas with increased surface roughness compared with the rest of the inside of the nozzle.